Separation of solids by varying the bulk density of a fluid separating medium

ABSTRACT

A method and apparatus for separating discrete solid articles according to their density is described. A fluid is placed in a vessel, and discrete solid articles having various densities are placed therein. The density of the fluid is altered to a value which will cause separation of the solid articles due to differences in buoyancy. The density of the fluid separating medium is preferably altered by injection of small gas bubbles or other fluid substances having a density differing from the first fluid placed in the vessel.

United States Patent 1191 Hsieh et al. July 2, 1974 [54] SEPARATION OF SOLIDS BY VARYING 2,697,384 12/1954 Craig et a]. /2l0/83 2,813,074 11/1957 Banks et al. 210 221 X A FLUID 3,202,285 8/1965 Williams.... 210/221 X 3,478,875 11/1969 Roberts 209/173 X [75] Inventors: Jack Jui-Chang Hsieh, Richland;

John C i e, i/ Harry Primary Examiner-John Adee Kornberg, Rlchlanda all of wash- Attorney, Agent, or Firm-Christensen, OConnor, [73] Assignee: The Battelle Development Gamson & Havelka Corporation, Columbus, Ohio [22 Filed: Feb. 4, 1970 5 h d d ABSTfRACT I rd met 0 an apparatus or separating screte s01 1211 Appl' 8467 articles according to their density is described A fluid is placed in a vessel, and discrete solid articles having [52] U5, Cl 209/173, 210/221, 209/10 various densities are placed therein. The density of the 51 int. Cl B03b 1/00 fluid is altered to a value which will cause Separation [58] Field of Search...2l0/83, 84, 221,361,315-319; of the solid articles due to differences in buoyancy. 209/2, 172, 172.5, 173, 10 The density of the fluid separating medium is preferably altered by injection of small gas bubbles or other [56] R f re e Cit d fluid substances having a density differing from the first placed in the vessel.

2,426,398 8/1947 Lathrod 210/173 7 Claims, 2 Drawing Figures FIELD OF THE INVENTION This invention relatesto a method and apparatus for separating a mixture of objects according to the density of the individual objects. This invention further relates to means for separating solids of varying densities by use of a fluid separating medium. More particularly, the present invention is concerned with a novel separating technique and apparatus in which solidarticles having slightly differing densities may be separated by use of a fluid separating medium in which the density is altered during the separating process in order to cause the solids of a higher density to be separated from the solids of a lower density due to positive buoyancy of the lower density solids and negativebuoyancy of the higher density solids.

PRIOR ART Froth flotation is a widely used technique in the chemical process and mineral industries to separate the more valuable materialsfrom the tailings or gangue. In

the froth flotation process finely ground materials are contacted with chemicals called promoters or collectors which render one portion of the particles air avid and water repellent. With vigorous agitation and aeration in the presence of a frother a layer of froth or foam forms at the top of the flotation machine. The portion of the particles which have become coated with the promoter or collector become attached to the air bubbles and rise to the surface where they may be collected by skimming techniques. The other portion of the material being treated is not attached to the bubbles and migrates to the bottom of the flotation apparatus. Depending upon the materials involved, the desired fraction either collects as a part of the froth at the top of the machine or as the unclerflow. Froth flotation techniques depend upon the surface characteristics of the materials being separated and the surface effects, including liquid repellency and interaction between the particles and air bubbles. These surface effects are altered by the use of frothers including pine oil, alcohols, and other surface characteristic modifiers and other activators, alkalinity regulators, depressants, dispersants and deflocculants. Density differences play a very minor role in froth flotation and, in fact, may inhibit separation in flotation processes. A material may be favorably affected by the surface characteristic modifiers, but fail to rise to the surface with the froth due to the high density of the material. F roth flotation is useful only in processes in which the materials are ground to a fine mesh size, such as less than about mesh. For larger size particles, density effects overcome the froth flotation action and no separation results.

For separation of larger objects such as produce including tomatoes and other vegetables, a buoyancy method of separation has been proposed in which the rate of ascent of the submerged objects in a brine solution is utilized to perfomi the separation. The produce is injected into a moving current of brine. The lower density objects surface first and are removed separately from the higher density objects. This process cannot differentiate between very slight differences in density and requires a source of brine.

OBJECTS or me INVENTION This invention has as one object the provision of a process and apparatus for separating relatively large mesh sized particles according to their density by use of a fluid separating medium. A further object of this invention is to provide a process and apparatus in which macroscopic sized solids are separated according to their densities by use of a fluid separating medium in which the effective density of the fluid separation medium is varied causing the higher density objects to have a negative buoyancy and the lower density objects to have a positive buoyancy.

Another object of this invention is to provide a method of and apparatus'for causing separation of solids according to their densities using a variable density fluid separating medium. A related object is to provide apparatus which causes a variation in the apparent density of the fluid separating medium so as to cause the lighter density fraction of the solid particles to float on the fluid separating medium and the heavier density particles to sink.

A still further object of this invention is to provide a convenient means to alter the effective density of a fluid. A further related object is to provide means to alter the effective density of fluids used in solids handling apparatus to effect a separation of the solids being treated in the apparatus simultaneously with the solids handling and transporting operation.

SUMMARY OF THE INVENTION These and other objects of this invention are achieved by providing a fluid column with means to alter the effective density of the fluid. The materials to be separated are placed in the fluid column and seek a position dictated by the buoyancy provided by the fluid. The effective density of the fluid separating medium is then changed to a value above that of the lower density fraction but below that of the higher density fraction of the materials being separated. The higher density fraction of the material being separated then sinks to the bottom of the fluid separating medium due to a negative buoyancy, and the lighter materials re main afloat. The fluid separating medium may have its effective density varied by one of a number of means including temperature variations, addition of chemicals soluble in the fluid, dilution with a lighter or heavier density fluid or injection of immiscible fluids into the fluid separating medium to alter the bulk density thereof.

IN THE DRAWINGS FIG. 1 shows a schematic side view of one embodiment of this invention being used to separate fruit or vegetables having differing densities;

FIG. 2 shows a schematic side view of another embodiment of this invention suitable for use as a garbage or refuse separator.

Referring particularly to the drawings, a water entry port 11 is shown at one side of tank 12. Air enters tank 12 through port 13 and is injected into the water through spargers 14. The objects being classified according to the density are shown entering the tank 12 from chute 16 and floating on the top of the water at 15. As the objects move across the surface of the water to the air-water mixture section 17, the heavier objects lose buoyancy and sink toward spargers 14. The lighter objects are removed from the surface of the air-water mixture section 17 by conveyor 20. The heavier objects move laterally under the current imposed by the bulk water flow in the apparatus until they are out of the airwater mixture zone 17. At that location positive buoyancy returns. The objects then resurface and are removed over weir 21. An inclined screen 25 is provided to reduce turbulence in the separation area 17 and to help move the objects having negative buoyancy in the separation area 17 toward weir 21.

The embodiment shown in FIG. 1 has been successfully used in separating frost-damaged fruit such as apples from the non-damaged counterpart due to the fact that frost-damaged fruits. have a different density. Other types of fruit, vegetables and similar objects of macroscopic size have been separated by use of this technique and apparatus. The density of many fruits and vegetables is a function of the relative ripeness of the individualproducts. Relatively greener produce has a markedly lower density than its riper counterpart. In addition, damaged fruit such as tomatoes and potatoes frequently have internal cavities which drastically reduce the density of the fruit. Use of the apparatus of this invention to sort fruit thus results in the unripe and damaged fruit being removed by conveyer 20 and the ripe product being removed over weir 21. If desired each product stream may be subjected to a second separation step to further classify the products as to density. Alternately, for more complete separation of the products into various grades according to the condition, ripeness and other density influencing factors, several air-liquid mixture zones can be provided in a single tank with a variety of air-liquid ratios presented to the objects being classified. Any number of product streams can thus be obtained by using multiple airliquid mixture zones.

One important feature of this separation technique and apparatus is that the produce being separated is contacted with the water or other liquid separating media. This contact will act to remove dirt or other loosely bound impurities from the produce. In addition, the produce may be conveniently treated by chemical additives or biochemically active substances added to the water. For example, the sprouting of potatoes may be inhibited by a chemical added to the water used in the separation process.

In FIG. 2 a second embodiment of this invention is presented. This embodiment of the invention permits the separation of various types of refuse materials on the basis of their density. In FIG. 2 is shown a single cell operation in tank 32 using a liquid such as water with air sparged into the lower portion of tank 32 through sparger 34 to produce an air-water mixture. The separating apparatus in this embodiment is made up of a tank 32 having a heavy material settling section 42 separated from the main part of the tank by baffle 38. A heavy material elevator 43 is provided to remove the heavy materials from the separator system. When water is used as a separating liquid, these materials would have a specific gravity exceeding 1.0. The refuse material itself is introduced into the separation apparatus through feeder 44. Water inlet 39 provides a flow of water through the apparatus to move floating material from its point of introduction into the separation apparatus toward the air-water mixture zone 46. The material that floats on water is shown at 47 being transported across to the air-water mixture zone 46. In zone 46 the effective density of the water is decreased by sparging of air into the water to a point at which a portion of the materials no longer has a positive buoyancy and is caused to sink into the air-water mixture 46. Air is introduced into the system through spargers 34 and caused to intimately mix with the water and rise through the water, thereby decreasing the effective density of the water. Materials which have a specific gravity less than the specific gravity of the bulk airwater mixture remain afloat and maybe removed by light material conveyor 50. The materials which have a specific gravity exceeding that of the bulk air-water mixture shown at 49 sink to a certain extent in the airwater mixture and are transported laterally in the separation apparatus by the bulk flow of water. When they reach the quiescent area of the separation chamber, shown at 51, they regain positive buoyancy and rise. to the surface of the water. From this point they may be removed by the flow of water over weir 53 onto separator screen 54. The water is drained and returned to the tank 32 through inlet 39.

The operation of the refuse or garbage separation embodiment ofthis invention proceeds as follows: the garbage or other refuse material is crushed or divided into a mesh size not exceeding approximately 1 inch for convenience of handling. However, larger materials can be introduced as desired. It is important to crush or grind the garbage and refuse in order to make sure that materials such as cans and bottles no longer retain their fluid holding capacity and will sink or float with materials having similar specific gravities. The refuse is dumped into the water from feeder 44. The heavy materials such as metals, glass, ceramics, rocks and other materials having a specific gravity exceeding 1.0 will immediately sink in heavy material removal section 42 and may be removed on heavy material conveyer 43. This material may, of course be subjected to a second separation step in which a heavier liquid such as an aqueous solution of calcium carbonate or other salt is used. I

The lighter materials 47, which float on water, are then transported laterally in the apparatus to the airwater mixture zone 46. At that point, materials which have a specific gravity of, for example, 0.7 to 1.0 are caused to lose buoyancy and sink due to the decreased effective density of the air-water mixture as compared to the adjacent water. Any material which remains at the surface, for example, that having a specific gravity less than 0.7, can then be removed by skimming or by removal on light material conveyer 50. The materials 49 having mid-range density sink in the air-water mixture and are moved laterally by the gross movement of the water in the system. When they leave the air-water mixture zone 46, into the quiescent zone 51, they regain their positive buoyancy and return to the water surface. These materials may then be removed by skimming or by water flow across weir 53.

Each of the product streams of the garbage or refuse separator may be further subjected to buoyancy separation such that a different fraction of the materials is separated by use of a different liquid or by use of various air to liquid ratios in the air-liquid mixture zone. Water is used as a convenient liquid because of its ready availability and low cost. However, liquids such as kerosene or other petroleum-based liquids or aqueous salt solutions may be used to perform the separation. Silicone oils having specific gravities up to about five groups with specific gravity ranges of, for example,

O 0.5, 0.5 0.7, 0.7 0.8, 0.8 1.0 and 1.0 and above. The various density ranges may be altered as desired, simply by changing the air toliquid ratio in the airliquid mixture zone to fit the particular refuse material being separated.

The following examples show the use of one embodiment of this invention shown in PK). 1 being used to separate damaged fruit from the good fruit. The examples are presented here for purposes of explanation only and are not intended as limitations on the scope of the invention in any way.

EXAMPLE I One problem in the apple industry is to detect and separate water-core apples from normal apples. Watercore apples rot within a short time after harvest and cause damage to the other apples when the apples are stored in bulk-type storage. Water-core apples are difficult, if not impossible to detect by inspection of the exterior of the apples. One usual characteristic of watercore apples is that they have a higher bulk density than the non water-core apples. Eleven apples, each having a perfect outward appearance, were floated upon water in a container and air was sparged into the bottom of the container by use of fritted glass spargers. Sufficient air was introduced into the bottom of the container to decrease the apparent density of the water to a value below that of one of the apples, as evidenced by its loss of buoyancy. The apples were then opened by cutting to check for water-core. The one apple which lost buoyancy was found to be a water-core apple and the remaining ten apples were sound.

EXAMPLE ll Twenty apples were treated as in Example I and two of the apples lost buoyancy at an effective density of the water equivalent to approximately 0.86. Upon cutting the apples it was found that one of the two apples which lost buoyancy was a water-core type apple. The other apple was sound and the other remaining 18 apples which retained buoyancy were found to be normal.

The apparatus shown in HO. 1 may also be used to separate materials such as bark from wood chips. One example of this use is discussed below.

EXAMPLE Ill types of solid items according to density. The separation technique of this invention may be used on various fruits, vegetables and other foodstuffs to separate the materials according to ripeness and defects. Typical ex amples of items which may be susceptible to this type i of separation technique would include potatoes, on-

ions, tomatoes, lima beans, dates, nuts, cherries, plums, pears, waterrnelons, and many other foodstuffs. Any material which has a density relatively close to that of a liquid can be separated from other articles having differing densities by this process and apparatus.

Citrus fruit growers will find the process and apparatus of this invention to be useful in separating the juicy, more valuable product suitable for fresh consumption from the pithy type of product which may be overripe and contain less of the desirable juicy characteristics. Peas, beans and other vegetables which are harvested without respect to the individual degree of ripeness of the individual vegetables may be separated based on the moisture content which directly influences the specific gravity or density of the vegetable. The process can, of course, be combined with a. washing of the vegetables or other articles being separated and possibly quick cooling of the vegetables to aid in preservation thereof. Similarly, sorting of eggs may be possible due to differences in densities of the eggs as they age.

Weed seeds can be easily separated from the more valuable grain and other type of seeds sought as prod ucts. Separations of types of grain seeds may also be possible if adequate differences exist in the respective densities of the types of grain.

For materials which have a specific gravity greater than 1.0, other fluids having specific gravity higher than that of water may be utilized as the basic fluid separating medium, for example, mercury could be used to separate lead particles from rocky material or other lighter density gangue type material if the leadis in, for example, shot form. Heavyorganic liquid such as chloroform or salt solutions such as aqueous calcium carbonate or sodium chloride could be used to separate solid materials having densities exceeding 1.2. Many other suitable liquids will occur to one skilled in the art for particular applications.

For articles having densities considerably less than 1.0, fluids with a density lighter than that of water may be utilized. For example, petroleum distillates, such as kerosene may be used to separate articles having densi ties in the range of approximately 0.6 to 0.75. Gaseous separating mediums could also be used for special applications. To use gases the density could be varied by variations in the pressure imposed on the container in which the separation process is taking place.

The air bubble generating apparatus used in each of the preferred embodiments discussed above comprises a partially sintered material such as a fritted glass disc, sintered stainless steel plate having gas porosity or other metallic or ceramic material having a porous structure. Stainless steel frits or copper based metal frits are preferable for use in aqueous systems to prevent oxidative degradation.

Many other methods of adjusting the effective density of a fluid by techniques well known in the art may be substituted for the methods described above. Typical method which may occur to those skilled in the art would include the entrainment of air in liquid jets positioned at the lower portion of the liquid tank; the dissolution of gases into liquids at a high pressure, which liquids are then introduced 'into the separation fluid; changing the effective density by electrolysis; and boiling the fluid to decrease the effective density thereof at certain locations.

In the usual application of this invention in which water or a heavier liquid is used as the separating medium, surface active agents which prevent the establishment of a permanent foam and aid in maintaining small gas bubble size may be used. Examples of such agents would include various low molecular weight alcohols and other additives well known in the art.

Systems in which two or more immiscible fluids are used may present the opportunity for complex, multiple splitseparations to take place within a single vessel by using various interfaces as the location at which articles of a particular density congregate.

While the present invention has been described with particular reference to specific examples, it will be apparent to those skilled in the art that various changes and modifications may be made thereto.

We claim as our invention:

1. A method for separating objects having a density exceeding a preselected density from objects having a density less than said preselected density comprising:

floating said objects on a liquid having a density exceeding said preselected density; sparging a gas into a portion of said liquid in sufficient quantities to lower the apparent density of said portion to said preselected density so that said objects having a density exceeding said preselected density sink and said objects having a density less than said preselected density remain afloat; and

separately removing said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said portion of liquid.

2. The method of claim 1 wherein any objects having a density exceeding said liquid prior to sparging said gas therein are separately removed in a preliminary separation step.

3. The method of claim 1 wherein said liquid is water.

4. The method of claim 1 wherein said gas is air. 5. A method of separating articles into a plurality of groupings according to density comprising the steps of:

introducing said articles into a chamber containing a liquid having a density exceeding that of at least some of said articles;

removing any of said articles which sink in said liquid from said chamber as one of said groupings;

sparging a gas into a portion of said liquid to adjust the effective density of said liquid to a value below the density of a second grouping of said articles but above the density of a third grouping of said articles so that said second grouping sinks leaving said third grouping afloat;

separately removing said third grouping from the surface of said liquid; and

causing said second grouping to move to an adjacent portion of said liquid being without sparged gas wherein said second grouping again floats to the top of said liquid for removal.

6. Apparatus for separating according to density a mixture of objects having a density exceeding a preselected density and objects having a density less than said preselected density comprising:

a vessel containing a liquid having a density such that at least part of said objects having a density exceeding said preselected density will float thereon;

means to place said mixture into said vessel;

means to adjust the effective density of one portion of said liquid to said preselected density whereby said objects having a density exceeding said preselected density sink into said liquid and said objects having a density less than said preselected density remain afloat, said means to adjust the effective density comprising gas sparging means positioned within said vessel; and,

means to separately remove said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said vessel, said means to separately remove said objects comprising a first removal means for collecting objects floating on said one portion of said liquid and means to move said objects having a density exceeding said preselected density to an adjacent portion of said liquid, being without gas sparged therein, wherein objects having a density exceeding said preselected density but less than the density of said liquid float to the liquid surface for removal from said liquid surface by a second removal means.

7. The apparatus of claim 6 and means to remove objects placed in said liquid which have a density exceeding that of said liquid prior to injecting said gas therein. 

1. A method for separating objects having a density exceeding a preselected density from objects having a density less than said preselected density comprising: floating said objects on a liquid having a density exceeding said preselected density; sparging a gas into a portion of said liquid in sufficient quantities to lower the apparent density of said portion to said preselected density so that said objects having a density exceeding said preselected density sink and said objects having a density less than said preselected density remain afloat; and separately removing said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said portion of liquid.
 2. The method of claim 1 wherein any objects having a density exceeding said liquid prior to sparging said gAs therein are separately removed in a preliminary separation step.
 3. The method of claim 1 wherein said liquid is water.
 4. The method of claim 1 wherein said gas is air.
 5. A method of separating articles into a plurality of groupings according to density comprising the steps of: introducing said articles into a chamber containing a liquid having a density exceeding that of at least some of said articles; removing any of said articles which sink in said liquid from said chamber as one of said groupings; sparging a gas into a portion of said liquid to adjust the effective density of said liquid to a value below the density of a second grouping of said articles but above the density of a third grouping of said articles so that said second grouping sinks leaving said third grouping afloat; separately removing said third grouping from the surface of said liquid; and causing said second grouping to move to an adjacent portion of said liquid being without sparged gas wherein said second grouping again floats to the top of said liquid for removal.
 6. Apparatus for separating according to density a mixture of objects having a density exceeding a preselected density and objects having a density less than said preselected density comprising: a vessel containing a liquid having a density such that at least part of said objects having a density exceeding said preselected density will float thereon; means to place said mixture into said vessel; means to adjust the effective density of one portion of said liquid to said preselected density whereby said objects having a density exceeding said preselected density sink into said liquid and said objects having a density less than said preselected density remain afloat, said means to adjust the effective density comprising gas sparging means positioned within said vessel; and, means to separately remove said objects having a density exceeding said preselected density and said objects having a density less than said preselected density from said vessel, said means to separately remove said objects comprising a first removal means for collecting objects floating on said one portion of said liquid and means to move said objects having a density exceeding said preselected density to an adjacent portion of said liquid, being without gas sparged therein, wherein objects having a density exceeding said preselected density but less than the density of said liquid float to the liquid surface for removal from said liquid surface by a second removal means.
 7. The apparatus of claim 6 and means to remove objects placed in said liquid which have a density exceeding that of said liquid prior to injecting said gas therein. 